Since you are manually trying to match a color to a swatch, I doubt that the result will be identical to the original anyway, but…
CMYK to RGB conversion formula
The R,G,B values are given in the range of 0…255.
The red (R) color is calculated from the cyan (C) and black (K) colors:
R = 255 × (1-C) × (1-K)
The green color (G) is calculated from the magenta (M) and black (K) colors:
G = 255 × (1-M) × (1-K)
The blue color (B) is calculated from the yellow (Y) and black (K) colors:
B = 255 × (1-Y) × (1-K)
source: CMYK to RGB conversion | color conversion
Seems like @pascal wrote a script a while ago as well…